In general electronic balances, an electromagnetic force generating device generates electromagnetic force which resists the displacement of the movable member in the sensor mechanism body due to the load of an object to be measured, and thus the load of the object to be measured is measured from the volume of the electromagnetic force that has been generated to make the displacement of the movable member in the sensor mechanism body zero.
An example of the sensor mechanism body where a movable member displaces due to the load of the object to be measured is formed of a Roberval mechanism (also referred to as parallel guide) provided with a fixed column fixed to an electronic balance base, a movable column (movable member) for transmitting the load of the object to be measured mounted on the weighing plate, and two beams parallel to each other for connecting the movable column to the fixed column. In the sensor mechanism bodies having such a Roberval mechanism formed in them, the displacement of the movable column due to the load of the object to be measured can be regulated in the vertical direction, and furthermore a shift error due to the location in which the object to be measured is placed on the weighing plate can be cancelled out.
Furthermore, the sensor mechanism bodies transmit the displacement of the movable column in the Roberval mechanism to the electromagnetic force generating device with a large lever ratio, and therefore are provided with a lever rockably supported by a fulcrum so that the displacement of the movable column in the Roberval mechanism connected to one end of the lever is transmitted to the electromagnetic force generating device connected to the other end of the lever.
Electronic balances provided with a sensor mechanism body where such a Roberval mechanism and a lever are integrally formed in one rectangular parallelepiped block made of metal (so-called single block type sensor mechanism body) have been known (see Patent Document 1).
FIG. 7 is a side view showing an example of a single block type sensor mechanism body, FIG. 8 is a side view showing an example of an electronic balance provided with the sensor mechanism body shown in FIG. 7, and FIG. 9 is a diagram schematically showing the structure of the electronic balance shown in FIG. 7.
The sensor mechanism body 201 is one rectangular parallelepiped block body made of an aluminum alloy and is formed of a Roberval mechanism R, a first lever 231, a second lever 232, and connection members 241, 242 for connecting the Roberval mechanism R to the first lever 231 and the second lever 232 through holes and slits which penetrate in the direction Y (direction of thickness).
The Roberval mechanism R is formed of a fixed column 210 which is fixed to an electronic balance base (not shown) through an attachment member 7 made of a separate member, a movable column 211 where a plate support 2a is fixed to the top, and two beams 212, 213 having flexible sections (hinge sections) 212a, 213a at the two ends. Thus, the movable column 211 and the fixed column 210 are connected through the two beams 212, 213 that are parallel to each other in the structure. Here, the weighing plate 2 on which an object to be measured is placed is placed on top of the plate support 2a. As a result, the displacement of the movable column 211 due to the load of the object to be measured is regulated in the direction Z (vertical direction).
The first lever 231 freely inclines around the center of the elastic first fulcrum 231a, and the second lever 232 freely inclines around the center of the elastic second fulcrum 232a. 
In addition, the movable column 211 in the Roberval mechanism R is connected to one end of the first lever 231 through the connection member 241, and the other end of the first lever 231 is connected to one end of the second lever 232 in the vicinity of the second fulcrum 232a through the connection member 242. The base section of a support member 6 made of a separate member is fixed by means of screws to the other end of the second lever 232 that is away from the second fulcrum 232a. As a result, the load of the object to be measured mounted on the weighing plate 2 makes the end of the support member 6 incline through the movable column 211, the connection member 241, the first lever 231, the connection member 242 and the second lever 232.
Such a displacement of the end of the support member 6 is detected by the displacement sensor 9 fixed to the electronic balance base. In addition, the force coil 3a in the electromagnetic force generating device 3 is fixed to the end of the support member 6. As a result, the amount of current flowing through the force coil 3a in the electromagnetic force generating device 3 is controlled by a servo mechanism (not shown) on the basis of the signal detected by the displacement sensor 9 so that the displacement of the end of the support member 6 is zero. In addition, the load of the object to be measured is measured from the amount of current flowing that is controlled by the servo mechanism.
Meanwhile, in electronic balances, the amount of current which makes equilibrium predetermined load may fluctuate day by day due to a change in the temperature or the like. As a result, there may be an error in the measured load of the object to be measured (results of measurement). Therefore, it is necessary to carry out correction using a weight for correction before the load of the object to be measured is measured in order to prevent the results of measurement from causing an error due to a change in the temperature or the like.
The weight for correction must be handled or maintained with special care, and therefore some electronic balances have an internal weight installed in advance (see Patent Document 2). In these electronic balances where an internal weight is installed, correction is carried out appropriately through the operation of a button by an operator or automatically carried, by a signal from a timer or a temperature sensor.
FIG. 10 is a diagram schematically showing the structure of an electronic balance where an internal weight is installed. The electronic balance 110 is provided with a main Roberval mechanism R1 for transmitting the load of the object to be measured mounted on the weighing plate 2 in the direction Z (vertical direction); a sub-Roberval mechanism R2 for transmitting the load of the internal weight 4 mounted on the engaging section 5; a switching mechanism (not shown) that can switch the position of the internal weight 4 between the one mounted on the engaging section 5 and the one not mounted on the engaging section 5; a first lever 31 rockably supported by the first fulcrum 31a; a second lever 32 rockably supported by the second fulcrum 32a; connection members 41, 142, 143 for connecting the main Roberval mechanism R1, the sub-Roberval mechanism R2, the first lever 31 and the second lever 32 to each other; and an electromagnetic force generating device 3 for generating an electromagnetic force.
In this electronic balance 110, the second movable column 21 in the sub-Roberval mechanism R2 is connected through a connection member 143 to one end of the second lever 32 which is on the opposite side of the other end of the second lever 2 connected to the electromagnetic force generating device 3 in order to carry out correction using the internal weight 4. The sub-Roberval mechanism R2 is formed of a common fixed column 10 fixed to the electronic balance base, the second movable column 21 to which the engaging section 5 is fixed, and two second beams 22, 23 having flexible sections (hinge sections) 22a, 23a at both ends. In addition, the second movable column 21 and the common fixed column 10 are connected through the two second beams 22, 23 which are parallel to each other in the structure. As a result, the displacement of the second movable column 21 due to the load of the internal weight 4 is regulated in the direction Z (vertical direction).
Thus, the switching mechanism does not place the internal weight 4 on the engaging section 5 when the load of an object to be measured is measured while the switching mechanism mounts the internal weight 4 on the engaging section 5 when correction is carried out so that the load of the internal weight 4 is transmitted to the second movable column 21. When the load of the internal weight 4 is transmitted to the second movable column 21, the second lever 232 inclines through the connection member 143.    Patent Document 1: Japanese Unexamined Patent Publication 2004-61257    Patent Document 2: Japanese Patent No. 3645372